This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A076543 #18 Apr 08 2025 18:36:52
%S A076543 1,3,6,2,7,13,20,12,3,13,24,12,25,39,54,38,55,37,56,36,57,79,102,78,
%T A076543 53,79,52,24,53,83,114,82,115,149,184,148,185,223,262,222,263,305,348,
%U A076543 304,259,305,352,304,255,205,256,204,257,203,258,202,259,317,376,316,377
%N A076543 a(n) = Sum_{k=1..n} k*sqf(k) where sqf(k)=1 if k is squarefree and sqf(k)=-1 otherwise.
%C A076543 Surprisingly, first 12 terms are also in A074170.
%H A076543 Harvey P. Dale, <a href="/A076543/b076543.txt">Table of n, a(n) for n = 1..1000</a>
%t A076543 Accumulate[Table[If[SquareFreeQ[n],n,-n],{n,70}]] (* _Harvey P. Dale_, Mar 18 2015 *)
%o A076543 (PARI) a(n) = sum(k = 1, n, if (issquarefree(k), k, -k)); \\ _Michel Marcus_, Oct 02 2013
%Y A076543 Cf. A074170.
%K A076543 easy,nonn
%O A076543 1,2
%A A076543 _Zak Seidov_, Oct 19 2002